Rolling magnesium alloy



United States ROLLING MAGNESIUM ALLOY Application September 28, 1956 Serial No. 612,888

'3 Claims. (Cl. 148-115) No Drawing.

The invention relates to an improved method of pro ducing rolled sheet of magnesium-base alloy. It more particularly concerns an improved method of producing rolled sheet in coiled form of a strain hardenable magnesium-base alloy.

Heretofore magnesium-base alloys of the strain hardenable type have been rolled into sheet to a limited extent from moderate size cast ingot, the sheet produced being short enough to be handled as individual pieces without coiling.

*Insofar as we are aware, there is no commercially available method by which magnesium-base alloys of the strain hardenable type can be rolled into a long sheet requiring a coiling operation which does not exhibit severe localized twinning or its concomitant discontinuous grain growth upon subsequently uncoiling the sheet to make it flat and heating it as required in proceeding with the usual bending and forming operations. The 10- calized twinning or discontinuous grain growth adversely affects the smoothness of the sheet as well as its mechanical properties. Accordingly, it is the principal object of the invention to provide an improved method of producing rolled sheet of the foregoing alloys which does not exhibit these adverse properties. Other objects and advantages will appear as the description of the invention proceeds.

The invention is predicated upon the discovery that by producing most of the reduction in thickness of the alloy by rolling at a temperature above the recrystallization temperature, then reducing the so-rolled metal to finish gauge in a single pass while reducing the temperature of the metal to somewhat above its minimum recrystallization temperature, following such single pass by an immediate quenching to lower the temperature of the metal to below that at which it recry'stallizes, and then coiling, the so-ooiled metal can be uncoiled so as to make it flat and submitted to all the usual forming operations requiring heating the metal, or limited amount of additional rolling, without producing localized twinning or discontinuous grain growth.

In carrying out the invention, cast ingot of a strain hardenable magnesium-base alloy is employed. Examples of these alloys and their AS'IM designations are: A231 (nominal composition: 3 percent Al, 1 percent Zn, 0.2 percent Mn, balance Mg); AZ61 (nominal composition: 6 percent. Al, 1 percent Zn, 0.15 percent Mn, balance Mg); HK31 (nominal composition: 3 percent Th, 0.7 percent Zr, balance Mg); HKll (nominal composition: .1 percent Th, 0.7 percent Zr, balance Mg); HM21.(nominal composition: 2 percent Th, 1 percent Mn, balance Mg); ZE41 (nominal composition: 4 percent Zn, 0.5 percent Zr, 0.4 R.E. (R.E. stands for metal of the rare earths), balance Mg); ZElO (nominal composition: 1 percent Zr, 0.12 percent R.E., balance Mg). The ingot is subjected to a solution heat treatment as by soaking in a furnace at a temperature at which substantially complete solution of the solid solution formtent "ice

ing alloying metal is obtained. A soaking temperature of 850 F. generally suflices. Soaking time varies with the volume of the ingot and its shape and generally ranges from about 10 to 20 hours including the time taken to heat the metal from room temperature to the solution heat treatment temperature. It is desirable to hold the ingot at the solution heat treatment temperature for at least 4 to 6 hours.

The solution heat treated ingot, while still hot, is rolled hot until the final pass is to be made. This hot rolling is accomplished by passing the metal between conventional rolls while the metal is at a temperature at which it can recrystallize using as many passes as necessary to obtain the desired thickness reduction prior to the final pass. 'Each hot pass produces a reduction in thickness of an amount less than that which would crack or fracture the metal as understood in the art of hot rolling a magnesium-base alloy. The number of hot passes will vary according to the initial thickness of the ingot and the ultimate thickness desired. As the hot rolling proceeds, the rolled metal lengthens. necessitates coiling the rolled metal if before the final pass suflicient space is not available to handle the sheet. In the event that coiling becomes necessary before the final pass the hot rolled metal is coiled while hot, uncoiled hot, and thereafter rolled hot. If desired, the coiled hot rolled metal may be set aside and allowed to cool before further rolling. Before again rolling the cooled coiled metal, the coil is heated to a temperature above its recrystallization temperature.

In accordance with the invention, the hot rolling, that is the rolling which is done at a temperature at which the metal is above its recrystallization temperature, is discontinued when the thickness to which the metal is thereby reduced is about 118 to 143 percent, or preferably about 125 percent, of the desired final thickness. The so-obtained metal is then rolled to its ultimate thickness in a single pass in a cold mill which provides conditions under which the metal emerges from the mill at a temperature slightly above its minimum recrystallization temperature. These conditions are that the hotrolled metal be at a temperature of about to 300 Fahrenheit degrees above its minimum recrystallization temperature, when about to enter the mill on the final pass, and that the mill, rolls be called so that the rolled metal will emerge from the mill at a suitably low temperature which is somewhat above its minimum recrystallization temperature.

The cooling of the rolls may be accomplished by flowing a cooled liquid lubricant over the rolls or through passages in the rolls, so that the metal, as it is reduced in thickness at the last pass to final gauge, loses heat to {the rolls and emerges from the rolls at a temperatures somewhat above that at which it recrystallizes, e.g. for" AZ31 about 275 F. The so-rolled metal just after it has emerged from the mill is then given an immediate quench to cool the metal to below its recrystallization temperature.

The required quenching may be accomplished by cooling with air, water, or other suitable fluid applied directly to the metal as it emerges from the mill rolls,

e.g. within 2 to 3 feet of the bite of the mill.

The so-quenched metal is air dried if necessary and then coiled dry as by winding it upon a mandrel which is suitably revolved, the temperature of the metal as it is coiled being below the recrystallization temperature. The minimum radius of the coil so-obtained should not be less than about 50 times the thickness of the metal so-coiled.

The so-coiled rolled metal is in the hard rolled con- .dition and exhibits high tensile and compressive strength This and other desirable properties. On being uncoiled to make'it flat and heated, as neededin forming operations, the metal is readily formable and does not exhibit undesirable twinning or discontinuous grain growth. As a consequence, articles formed or drawn 'from the metal taken from the coil exhibit a smooth surface free from evidence of twinning or sporadic grain growth.

The following exampleis illustrative of the practice of the invention:

A scalped cast ingot 12 inches thick, 40' inches wide, and 76 inches long weighing 2250 poundsof'a mag nesium-base alloy having the nominal composition of 3 percent Al, 1 percent Zn, L2 percent'Mn, the balance being Mg, was solution heat treated at 850 to900 F."

for about 16 hours. The so-treatedJingot, thereby heated to about 850 -F:, was then subjected to hot rollin'gon a breakdown mill with 27 inch diameter work roll's. In the first passes, the ingot was passedbetween the'rolls sideways which' widened the ingot to5l inches and'atthe same time reduced its thicknessto 9.4 inches. A roll lubricant of a' percent water solution ofa liquid. polyglycol wasapplied to the rolls the temperature "of which was estimatedxto be about 1 40'F. Theroll'surface speed was about 600 feet per minute.

rolls and sent between the rolls in the endways direction in 18 passes which reduced the thickness to 0.18 inch. During these passes the temperature of the rolled metal fell to about 550 F. and on leaving the mill' at the 18th pass, the metal while still above 550 'F., was led to the coiler having a mandrel 24 inches in diameter and thereon coiled. During the foregoing passes'the'metal widened about one-half inch and increased in length to The coiled metal while still hot was placed in an oven at 650 F. to await further rolling in 3 about 320 feet.

more passes to final gauge on a finishing mill. The first. two of these passes were hot rolling passes at temperatures between 540 and 650 F., the metallbeing uncoiled as it entered the rolls and coiled again (mandrel diameter 24 inches) while hot, except on the 3rd pass, at it Uncoiled samples of the so-rolled metal of the above example heated to about 600 F., prior'to submitting them to a forming operation, exhibited a smooth surface free from evidence of twinning or sporadic grain growth. The sheet was easily formable by bending and drawing without cracking.-

We claim:

1. The method ofreducing-the thickness -of a rolled article of a strain hardenable magnesium-base alloy in following such single pass by animmediate quenching.

so as to lowerthe temperature of themetal below-that at which it'recrystallizes, and then coiling the 'so-quenched' metal, the minimum radius of the coil so-produced being rolled The so-rolled ingot was then turned 90 int-he plane of the axes ofthe" was accomplished by transferring heat from the metal to the rolls by cooling the rolls to a temperature between about 100 and 120 F. and by adjusting the speed of the rolls to about 500 feet per minute surface speed so that the metal was retained in the bite .of the rolls long enough to bring about the desired temperature lowering.

As the so-cooled and rolled metal emerged from the mill, it was quenched as by cooling with water to a temperature below 260 F. The as-cooled metal was dried as by directing an air wipe to the metal and then coiled dry, using a 24 inch diameter mandrel, while at a temperature between about 220 and 260 F. The uncoiled length of the s c-produced sheet was about 700feet.

Samples of the so-rolled metal uncoiled from the coil exhibited the following properties:

Strength in 1,000 p.s.i. Percent Condition UTS Tits 1 OYS 1 as rolled 42 000 annealed percent deviation from the modulus line 3 1 hour 265 F.

E 111 2 inches the recrystallized state to a preselected final thickness which comprises rolling between the'rolls of a'mill the article at a temperature above the recrystallizationtemperature so as to reduce its thickness tobetWeen 118 ,and 143 percentof the desired -final-"thickness,rolling the so-rolled metal to the desired final thickness while reducing the temperature of the-metal beingrolled by contact with the rolls of the mill to above and near its minimum. recrystallization temperature in a single pass,

not less than about 50 times the thickness of the metal.

2. The method of reducing the thickness ofa' rolled article of a strain hardenable'maguesium-base alloy in the recrystallized state to a preselected final thickness which comprises rolling between the rolls'of a mill the article at a temperature above the recrystallization temperature so as to reduce its 'thicknessto between 118 and 143 percent of the desired final thickness, cooling the so-obtained hot rolled metal to'a temperature about to 300 Fahrenheit degrees above its minimum recrystallization temperature, rolling the so-rolled metal to the desired final thickness while reducing the tempera ture of the metal being rolled by contact withthe rolls of the mill to above and near its minimum recrystallization temperaturein a single pass, following such single pass by an immediate quenching so as to'lowerthe temperature of the metal below that atwhichit recrystallizes,

and then coiling the so-quenched metal, the minimum radius of the coil so-producedbeing not less than about 50 times the thickness of the rolled' metal;

3. The method of reducing the thickness of a rolled" article of a strain hardenable magnesium-base alloy in. the recrystallized state to a preselected final thickness which comprises rolling between the rolls of a mill the article at a temperature above the recrystallization temperature so as to reduce its thickness to between 118' and 143 percent of the desired final thickness, cooling the so-obtained hot rolled metal to a temperature about 100 and 300 Fahrenheit degrees above its minimum re-I crystallization temperature, rolling the so-cooled metal to the desired final thickness while reducing the temperature of the metal beingrolled by contact with 'the rolls of the mill to above and near its minimum recrystallization temperature in a single pass, following such single pass by applying a quench of water so as to lower the temperature of the metal below that at which it recrystallizes, and then coiling the so-quenched metal, the

minimum radius of the coil so-produced beingnotless than about 5 0 times the thickness of the-ro1led.metal'.'

References Cited in the file of this patent UNITED STATES PATENTS 2,029,728 Lowry et al. -Feb. 4, 1936i v 2,294,648 Ansel et al. Sept. 1, 1942 2,314,010 McDonald Mar. 16, 1943 OTHER REFERENCES Iron Age, page 45,- Mar.. 18, 1943. Introduction to Mg and its Alloys, John Alico;(Zifi-" Davis Publishing Co.), pages 82- 86, 1945.-

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No., 2 934 462 April 26 1960 Hugo A, Barbian et a1,

It is hereby certified that error aopears in the-printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2 line 455 for "called" read cooled a Signed and sealed this 29th day of November 1960.

(SEAL) Attest:

KARL Ho AXLINE ROBERT C. WATSON Attesting Ofiicer Commissioner of Patents 

1. THE METHOD OF REDUCING THE THICKNESS OF A ROLLED ARTICLE OF A STRAIN HARDENABLE MAGNESIUM-BASE ALLOY IN THE RECRYSTALLIZED STATE TO A PRESELECTED FINAL THICKNESS WHICH COMPRISES ROLLING BETWEEN THE ROLLS OF A MILL THE ARTICLE AT A TEMPERATURE ABOVE THE RECRYSTALLIZATION TEMPERATURE SO AS TO REDUCE ITS THICKNESS TO BETWEEN 118 AND 143 PERCENT OF THE DESIRED FINAL THICKNESS, ROLLING THE SO-ROLLED METAL TO THE DESIRED FINAL THICKNESS WHILE REDUCING THE TEMPERATURE OF THE META BEING ROLLED BY CONTACT WITH ROLLS OF THE MILL TO ABOVE AND NEAR ITS MINIMUM CRYSTALLIZATION TEMPERATURE IN A SINGLE PASS, FOLLOWING SUCH SINGLE PASS BY AN IMMEDIATE QUENCHING SO AS TO LOWER THE TEMPERATURE OF THE METAL BELOW THAT AT WHICH IT RECRYSTALLIZES, AND THEN COILING THE SO-QUENCHED METAL, THE MINIMUM RADIUS OF THE COIL SO PRODUCED BEING NOT LESS THAN ABOUT 50 TIMES THE THICKNESS OF THE ROLLED METAL. 